The present invention concerns a scanning optical device provided in a laser beam printer or the like for scanning a light beam.
A scanning optical device includes a light source such as a semiconductor laser, a polygon mirror for deflecting and scanning a light beam emitted from the light source, and an f.theta. lens that converges the light beam onto an image surface such as a photoconductive drum to form a scanning beam spot. Additionally, a cylindrical lens is provided between the light source and the polygon mirror, to form a linear image in the vicinity of the reflecting surface of the polygonal mirror.
Conventionally, the cylindrical lens is used to compensate for shape defects of the beam spot due to misalignment of a lens in the optical system. Accordingly, in a conventional scanning optical device, the cylindrical lens is adjustable, for example, slidable in an auxiliary scanning direction and may also be rotatable about the optical axis. The shape of the beam spot is measured at positions adjacent to the image surface during assembly. This measurement and adjustment are repeated until the measured shape of the beam spot is acceptable.
Further, a bow of the scanning line (i.e. when the scanning line is curved) in the auxiliary scanning direction should also be compensated for in a scanning optical device having relatively high resolution. Since the bow of the scanning line occurs due to the misalignment of a lens in the auxiliary scanning direction, it can be compensated for by adjusting the cylindrical lens in the auxiliary scanning direction. Although the shift of the cylindrical lens may compensate for the bow of the scanning line, the shape of the beam spot will also be varied by this movement of the cylindrical lens and the variation of shape caused is larger than the amount of compensation provided for the bow of the scanning line.